The Sky This Week, 2017 May 9 - 16

The eternal chase

Jupiter & its moon Io, imaged 2017 April 27
from Alexandria, Virginia

The Moon moves into the morning sky this week as she slides into the southern summer constellations. Full Moon occurs on the 10th at 5:42 pm Eastern Daylight Time. May’s Full Moon is popularly known as the Milk Moon, Flower Moon, or Corn Planting Moon in various sky traditions. Look for the Moon to the north of the "head" of Scorpius during the morning hours of the 12th. She’ll be just under five degrees northeast of Saturn in the pre-dawn sky on the 14th.

If you’re following the changing star patterns through the course of the year, this is an interesting time to look at the horizons during the course of the evening. Start by looking southwest at around 9:00 pm. You’ll see the bright stars of Orion, arguably the brightest constellation in the sky, sinking below the horizon. Orion’s signature star, the red-tinted Betelgeuse, is the last star in the group to disappear, setting shortly after 10:00 pm. As Betelgeuse sets, look to the southeast for another ruddy beacon to appear. This is Antares, the brightest star in the constellation of Scorpius. These two luminaries share common characteristics and a connection in the sky lore surrounding their parent star patterns. Both stars owe their red hue to their physical nature as highly evolved "red supergiant" stars. They have exhausted the main supply of nuclear-fusible material in their cores and are fusing heavier elements in shells surrounding an ever-increasing sphere of nuclear "ash". This causes their outer layers to expand dramatically. If they were placed at the Sun’s position in our solar system their outer layers would swallow up the orbit of the Earth and possibly expand to the orbit of Mars. In fact, Betelgeuse was the first star to have its apparent diameter measured back in 1920, and it was also the first star to be directly imaged using interferometry. Antares is a similar star, with comparable mass and evolutionary history. Both stars are likely supernova candidates and are likely to go out in blazes of glory within the next few hundred thousand years. In mythology, Orion, a demi-god who claimed dominion over all animals on the Earth, boasted of his hunting prowess and his ability to kill any creature. Gaia, the personification of Mother Earth, created the lowly scorpion to punish Orion for his swagger. The creature stung Orion in the heel, killing him. This in turn caused Artemis, goddess of the hunt, to ask her father Zeus to place Orion in the sky. Gaia still had the last laugh, placing the scorpion in the sky as well to pursue Orion around the heavens forever. This is the reason why you will never see the two constellations in the sky simultaneously!

As you watch Orion set, look farther to the north along the western horizon for another ruddy glimmer in the deepening twilight. This is Mars, which has been a fixture in this part of the sky for the past several months. The red planet is pressing eastward against the stars and is presently moving between the bright stars Aldebaran in Taurus and El Nath in Auriga.

Jupiter dominates the evening sky, appearing shortly after sunset high in the southeast. Old Jove is the fourth-brightest object in the sky after the Sun, Moon, and Venus, and you will have no trouble spotting him, even through thin clouds. After the Moon, Jupiter is the most rewarding solar system target for the small telescope. You can spend an entire evening watching the changes in the positions of his four bright Galilean moons with a small telescope. A six-inch or larger instrument will allow you to see changes on the planet itself as Jupiter’s rapid rotation brings new atmospheric features into view over just a few minutes. On the evening of the 11th you can watch the famous Great Red Spot rotate off the planet’s disc as its innermost large moon Io begins to drag its shadow over the cloud tops. You’ll get another good view of the Red Spot on the evening of the 13th.

Saturn now rises at around 11:00 pm, and he’s best seen in the wee hours before morning twilight, crossing the meridian shortly before 4:00 am. The planet’s signature rings are well-presented to our line of sight, and you can see them with almost any kind of optical aid.

Venus shines with her dazzling glow in the morning twilight, rising shortly after 4:00 am. She should remain visible until Old Sol comes up about two hours later.